In the semiconductor industry, establishing the integrity of a via hole or aperture, which provides a connection between two separate electrically conducting planes, is a major problem. In the last few years, the size of these via holes or apertures has shrunk substantially, and verifying that the aperture bottom is clean and contains no material that would interfere with flow of charge therethrough has become much more difficult.
As illustrated in FIG. 1, a via aperture 11 is made by etching of a portion of a layer 13 of oxide, nitride or other electrical insulating or dielectric material, where the depth of the aperture may be 0.1-5 .mu.m and the diameter thereof may be 0.1-5 .mu.m or even smaller. Because of the high aspect ratio and steep walls of the aperture, it is often difficult to determine when the oxide material is fully penetrated for the first time or whether any residue 15 remains at the aperture bottom. Overetching of the oxide is usually undesirable and can give rise to many other problems. A residue of the oxide or etchant or any other material of thickness of as little as 50 .ANG. can greatly increase the contact resistance, thereby decreasing the reliability of the contact or even eliminating any reasonable conductivity associated with the path defined by the aperture. It is very difficult to inspect or confirm, by any presently known method, the presence of such a residue layer at the aperture bottom. Where an optical method of inspection is used, the small diameter of the hole dictates that only high numerical apertures be used. However, the high aspect ratio of the aperture then produces interference from the dielectric material that surrounds and defines the aperture, and it often becomes impossible to make an accurate reflectance or ellipsometric measurement.
Heimann et al. in U.S. Pat. No. 4,680,084, discloses determining the thickness of an etched semiconductor region by monitoring the intensity of light reflected from the etched layer, where an opaque substrate layer underlies or overlies the etched layer. The opaque layer suppresses the appearance of undesired interference effects in monitoring reflections from the material present in the etched layer and in the layer that would be exposed by a complete etch.
U.S. Pat. No. 4,725,332, issued to Spohr, discloses a method for determining the diameter of one or more microholes formed in a sheet of material. This technique uses a test region that is spaced apart from the fabrication region on the sheet of material. The inventor asserts that, at a porosity of around 0.7 wherein approximately 30 percent of the sheet material remains after formation of see-through holes therein, the electrical and optical characteristics of the remaining material change markedly. Assuming this to be so the technique appears to be limited to determining whether porosity in the sheet, due to appearance of see-through microholes therein, has reached or exceeded 70 percent.
Zingher, in U.S. Pat. Nos. 4,443,278 and 4,578,279, discloses electrical inspection of apertures, and the material left therein, in multilayer ceramic circuits, using techniques such as secondary emission or electrical conductivity measurements on any residue material that remains. The inventor also discloses use of a form of secondary electron emission that uses ultraviolet light as a probe and relies upon some unspecified photoelectric action in the target material (residue). Another method relies upon unspecified use of a light beam and an electron or ion beam in tandem. It is unclear how the Zingher invention would be applied to determining the thickness of dielectric layer residue at the bottom of an aperture.
Thus far, the only reliable method for determination of the presence or absence of any residue at an aperture bottom has been by cleavage of the dielectric material through one of the via apertures and inspection thereof using a scanning electron microscope. This method is very tedious and slow, and this approach interrupts the semiconductor fabrication process. What is needed is a technique for determining the presence or absence of residue at the aperture bottom in a dielectric material, where the technique allows accurate, in-process determination of the interference by such residue with the flow of charge through a channel defined by the aperture. Preferably, the method should also be usable with a variety of dielectric materials and aperture depths and diameters.